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<p>C 语言标准精确指定了 C 语言程序的<a href="https://zh.cppreference.com/mwiki/index.php?title=c/language/as_if&amp;action=edit&amp;redlink=1" class="new" title="c/language/as if（页面不存在）">可观察行为</a>，除了下列分类之一：
</p>
<ul><li> <i>未定义行为</i> - 程序的该行为没有限制。未定义行为的例子是越过数组边界的访问、有符号整数溢出、空指针解引用、在表达式中<a href="eval_order.html" title="c/language/eval order">超过一次</a>修改标量而其中无顺序点、通过不同类型的指针访问对象，等等。编译器不要求诊断未定义行为（尽管多数简单情形是得到诊断的），且编译后的程序不要求做任何有意义的事。
</li></ul>
<ul><li> <i>未指定行为</i> - 容许二种或多种行为，且不要求实现规范每种行为。例如，<a href="eval_order.html" title="c/language/eval order">求值顺序</a>、同样的<a href="string_literal.html" title="c/language/string literal">字符串字面量</a>是否有别，等。每个未指定行为导致一组合法结果之一，并且可以在同一程序中重复时产生不同结果。
</li></ul>
<ul><li> <i>实现定义行为</i> - 在未指定行为之上，实现规范了如何选择。例如，字节中的位数，或有符号整数右移是算术还是逻辑。
</li></ul>
<ul><li> <i>本地环境限定行为</i> - 依赖于<a href="../locale/setlocale.html" title="c/locale/setlocale">当前选择的本地环境</a>的实现定义行为。例如， <span class="t-lc"><a href="../string/byte/islower.html" title="c/string/byte/islower">islower</a></span> 对任何 26 个小写拉丁字母外的字符是否返回 true。
</li></ul>
<p>（注意：<a href="conformance.html" title="c/language/conformance">严格遵从</a>的程序不依赖任何未指定、未定义或实现定义行为）
</p><p>要求编译器对违背任何 C 语法规则或语义约束的任何程序发布诊断消息（错误或警告），即使其行为被指定为未定义或实现定义，或者编译器可提供语言扩展以允许此种程序被接受。另外，不要求对未定义行为诊断。
</p>
<h3><span class="mw-headline" id="UB_.E4.B8.8E.E4.BC.98.E5.8C.96">UB 与优化</span></h3>
<p>因为正确的 C 程序是没有未定义行为的，编译器可以在启用优化的条件下编译确实有 UB 的程序时，生成不期待的结果：
</p><p>例如，
</p>
<h4><span class="mw-headline" id=".E6.9C.89.E7.AC.A6.E5.8F.B7.E6.BA.A2.E5.87.BA">有符号溢出</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> foo<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">return</span> x<span class="sy2">+</span><span class="nu0">1</span> <span class="sy1">&gt;</span> x<span class="sy4">;</span> <span class="co1">// 真或为有符号溢出导致的 UB</span>
<span class="br0">}</span></pre></div></div>
<p>可以编译成（<a rel="nofollow" class="external text" href="https://godbolt.org/z/D249FL">演示</a>）
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">foo<span class="sy4">:</span>
        movl    $<span class="nu19">1</span>, <span class="sy2">%</span>eax
        ret</pre></div></div>
<h4><span class="mw-headline" id=".E8.B6.8A.E7.95.8C.E8.AE.BF.E9.97.AE">越界访问</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> table<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> exists_in_table<span class="br0">(</span><span class="kw4">int</span> v<span class="br0">)</span>
<span class="br0">{</span>
    <span class="co1">// 在最初的 4 个迭代中返回真或因为越界访问 UB</span>
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1">&lt;=</span> <span class="nu0">4</span><span class="sy4">;</span> i<span class="sy2">++</span><span class="br0">)</span> <span class="br0">{</span>
        <span class="kw1">if</span> <span class="br0">(</span>table<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1">==</span> v<span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="br0">}</span>
    <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可以编译成（<a rel="nofollow" class="external text" href="https://godbolt.org/z/qoD2uP">演示</a>）
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">exists_in_table<span class="sy4">:</span>
        movl    $<span class="nu19">1</span>, <span class="sy2">%</span>eax
        ret</pre></div></div>
<h4><span class="mw-headline" id=".E6.9C.AA.E5.88.9D.E5.A7.8B.E5.8C.96.E6.A0.87.E9.87.8F">未初始化标量</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">_Bool p<span class="sy4">;</span> <span class="co1">// 未初始化局部变量</span>
<span class="kw1">if</span><span class="br0">(</span>p<span class="br0">)</span> <span class="co1">// 访问未初始化标量是 UB</span>
    <a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"p is true"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>p<span class="br0">)</span> <span class="co1">// 访问未初始化标量是 UB</span>
    <a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"p is false"</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>可能产生下列输出（可在一个旧版本 gcc 观察到）：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">p is <span class="kw2">true</span>
p is <span class="kw2">false</span></pre></div></div>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><a href="../types/size_t.html"><span class="kw100">size_t</span></a> f<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span>
<span class="br0">{</span>
    <a href="../types/size_t.html"><span class="kw100">size_t</span></a> a<span class="sy4">;</span>
    <span class="kw1">if</span><span class="br0">(</span>x<span class="br0">)</span> <span class="co1">// x为非零或UB</span>
        a <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>
    <span class="kw1">return</span> a<span class="sy4">;</span> 
<span class="br0">}</span></pre></div></div>
</div>
<p>可以编译成（<a rel="nofollow" class="external text" href="https://godbolt.org/z/u9lBlY">演示</a>）
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">f<span class="sy4">:</span>
        mov     eax, <span class="nu0">42</span>
        ret</pre></div></div>
<h4><span class="mw-headline" id=".E9.9D.9E.E6.B3.95.E6.A0.87.E9.87.8F">非法标量</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span>
  _Bool b <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
  <span class="kw4">unsigned</span> <span class="kw4">char</span><span class="sy2">*</span> p <span class="sy1">=</span><span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span><span class="sy3">&amp;</span>b<span class="sy4">;</span>
  <span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">10</span><span class="sy4">;</span>
  <span class="co1">// 从 b 读取现在是 UB</span>
  <span class="kw1">return</span> b <span class="sy1">==</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可编译成（<a rel="nofollow" class="external text" href="https://gcc.godbolt.org/z/JGG6uI">演示</a>）
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">f<span class="br0">(</span><span class="br0">)</span><span class="sy4">:</span>
        movl    $<span class="nu19">11</span>, <span class="sy2">%</span>eax
        ret</pre></div></div>
<h4><span class="mw-headline" id=".E7.A9.BA.E6.8C.87.E9.92.88.E8.A7.A3.E5.BC.95.E7.94.A8">空指针解引用</span></h4>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> foo<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span> p<span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">int</span> x <span class="sy1">=</span> <span class="sy2">*</span>p<span class="sy4">;</span>
    <span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>p<span class="br0">)</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="co1">// 为上述 UB ，或绝不采用此分支</span>
    <span class="kw1">else</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">int</span> bar<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="sy4">;</span>
    <span class="kw1">return</span> <span class="sy2">*</span>p<span class="sy4">;</span>       <span class="co1">// 无条件 UB</span>
<span class="br0">}</span></pre></div></div>
<p>可以编译成（ <a rel="nofollow" class="external text" href="https://godbolt.org/z/W6nz9j">foo 用 gcc ， bar 用 clang</a> ）
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1">foo<span class="sy4">:</span>
        xorl    <span class="sy2">%</span>eax, <span class="sy2">%</span>eax
        ret
bar<span class="sy4">:</span>
        retq</pre></div></div>
<h4><span class="mw-headline" id=".E8.AE.BF.E9.97.AE.E4.BC.A0.E9.80.92.E7.BB.99_realloc_.E7.9A.84.E6.8C.87.E9.92.88">访问传递给 realloc 的指针</span></h4>
<div class="t-example"><p>选择 Clang 以观察示出的输出</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdio.h&gt;</span>
<span class="co2">#include &lt;stdlib.h&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">int</span> <span class="sy2">*</span>p <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><a href="../memory/malloc.html"><span class="kw403">malloc</span></a><span class="br0">(</span><span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">int</span> <span class="sy2">*</span>q <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><a href="../memory/realloc.html"><span class="kw405">realloc</span></a><span class="br0">(</span>p, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 访问传递给 realloc 的指针是 UB</span>
    <span class="sy2">*</span>q <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
    <span class="kw1">if</span> <span class="br0">(</span>p <span class="sy1">==</span> q<span class="br0">)</span> <span class="co1">// 访问传递给 realloc 的指针是 UB</span>
        <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%d%d<span class="es1">\n</span>"</span>, <span class="sy2">*</span>p, <span class="sy2">*</span>q<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">12</pre></div></div> 
</div>
<h4><span class="mw-headline" id=".E6.97.A0.E5.89.AF.E6.95.88.E5.BA.94.E7.9A.84.E6.97.A0.E9.99.90.E5.BE.AA.E7.8E.AF">无副效应的无限循环</span></h4>
<div class="t-example"><p>选择 Clang 以观察示出的输出</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdio.h&gt;</span>
 
<span class="kw4">int</span> fermat<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
  <span class="kw4">const</span> <span class="kw4">int</span> MAX <span class="sy1">=</span> <span class="nu0">1000</span><span class="sy4">;</span>
  <span class="kw4">int</span> a<span class="sy1">=</span><span class="nu0">1</span>,b<span class="sy1">=</span><span class="nu0">1</span>,c<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span>
  <span class="co1">// 无副效应的无限循环是 UB</span>
  <span class="kw1">while</span> <span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">if</span> <span class="br0">(</span><span class="br0">(</span><span class="br0">(</span>a<span class="sy2">*</span>a<span class="sy2">*</span>a<span class="br0">)</span> <span class="sy1">==</span> <span class="br0">(</span><span class="br0">(</span>b<span class="sy2">*</span>b<span class="sy2">*</span>b<span class="br0">)</span><span class="sy2">+</span><span class="br0">(</span>c<span class="sy2">*</span>c<span class="sy2">*</span>c<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
    a<span class="sy2">++</span><span class="sy4">;</span>
    <span class="kw1">if</span> <span class="br0">(</span>a<span class="sy1">&gt;</span>MAX<span class="br0">)</span> <span class="br0">{</span> a<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span> b<span class="sy2">++</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw1">if</span> <span class="br0">(</span>b<span class="sy1">&gt;</span>MAX<span class="br0">)</span> <span class="br0">{</span> b<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span> c<span class="sy2">++</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw1">if</span> <span class="br0">(</span>c<span class="sy1">&gt;</span>MAX<span class="br0">)</span> <span class="br0">{</span> c<span class="sy1">=</span><span class="nu0">1</span><span class="sy4">;</span><span class="br0">}</span>
  <span class="br0">}</span>
  <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span>
  <span class="kw1">if</span> <span class="br0">(</span>fermat<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
    <a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"Fermat's Last Theorem has been disproved."</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="kw1">else</span>
    <a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"Fermat's Last Theorem has not been disproved."</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Fermat's Last Theorem has been disproved.</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 标准（ISO/IEC 9899:2011）：
</li></ul>
<dl><dd><ul><li> 3.4 Behavior (p: 3-4)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 4/2 Undefined behavior (p: 8)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 标准（ISO/IEC 9899:1999）：
</li></ul>
<dl><dd><ul><li> 3.4 Behavior (p: 3-4)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 4/2 Undefined behavior (p: 7)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 标准（ISO/IEC 9899:1990）： 
</li></ul>
<dl><dd><ul><li> 1.6 DEFINITIONS OF TERMS 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.A4.96.E9.83.A8.E9.93.BE.E6.8E.A5">外部链接</span></h3>
<ul><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html">每个 C 程序员都该知道关于未定义行为的事 #1/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html">每个 C 程序员都该知道关于未定义行为的事 #2/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html">每个 C 程序员都该知道关于未定义行为的事 #3/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blogs.msdn.com/b/oldnewthing/archive/2014/06/27/10537746.aspx">未定义行为能导致时间旅行（在所有事项中，时间旅行可是最惊人的）</a>
</li><li> <a rel="nofollow" class="external text" href="http://www.cs.utah.edu/~regehr/papers/overflow12.pdf">了解 C/C++ 中的整数溢出</a>
</li><li> <a rel="nofollow" class="external text" href="http://kukuruku.co/hub/cpp/undefined-behavior-and-fermats-last-theorem">未定义行为及费马最后定理</a>
</li><li> <a rel="nofollow" class="external text" href="http://lwn.net/Articles/342330/">空指针的趣事， part 1</a> （ Linux 2.6.30 中由空指针解引用导致的未定义行为所引发的局部滥用）
</li></ul>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table></div>
</div>

<!-- 
NewPP limit report
Preprocessor visited node count: 1085/1000000
Preprocessor generated node count: 3582/1000000
Post‐expand include size: 27246/2097152 bytes
Template argument size: 6165/2097152 bytes
Highest expansion depth: 13/40
Expensive parser function count: 0/100
-->

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